1. Field of the Invention:
The present invention relates to a tin mill black plate for canmaking, such sheet having temper rolling degrees of T1-T6 or DR 8-DR 10. This invention also relates to a method for manufacturing the sheet.
More particularly, the present invention relates to a plated steel sheet for making a three-piece can, the sheet having small thickness, high strength and excellent welding properties. It further relates to a plated steel sheet for making a two-piece can, the sheet having small thickness and excellent drawability. This invention further relates to a method for manufacturing the sheets.
2. Description of the Related Art:
(1) Types of Cans
There are two types of cans made from steel sheet, namely, two-piece cans and three-piece cans. The former can be further classified as SDC (Shallow-Drawn Cans), DRDC (Drawn & Redrawn Cans), DTRC (Drawn & Thin Redrawn Cans), and DWIC (Drawn & Wall Ironed Cans).
(2) Types of Steel Sheets
These cans are manufactured by processes such as deep-drawing, ironing, bending, stretching and welding etc. appropriately tin-coated black plate. The tin mill black plate can be classified, depending on the properties and methods of making the can to be manufactured, into temper degrees of T1-T6 or DR8-DR10. Those black plates having temper degrees of T1-T3 are called soft-temper tin mill black plates while those of T4-T6 are called hard-temper tin mill black plates; both types are made by temper rolling a cold rolled steel sheet once. Meanwhile, classes DR8-DR10 are called DR black plate, manufactured by rolling with a large rolling reduction to the cold rolled steel sheet.
Conventionally, these steel sheets have been manufactured by preparing parent materials having originally different composition, and individually varying the conditions for the hot rolling, the cold rolling, and the annealing etc. for each of them, due to their fundamentally different requirements for strength and processing properties and the like. As a result, the processes have had to be changed each time to meet the requirements for the desired sheet, causing the manufacturing cost to be relatively increased.
(3) Steel Sheet for Three-Piece Cans and Its Problems
Steel sheet for cans must be thin with high strength to reduce cost. The three-piece can is not an exception, but is further required to have high-speed welding properties. In particular, it must provide a high-quality seam by electric seam welding method at more than 70 MPM of welding speed.
However, in the conventional art, reducing the thickness would lead to narrowing of the available welding current range. This is disadvantageous since when a relatively high welding current is supplied, splashing takes place during the welding process to undesirably increase the hardness of the welded portion. As a result, in flange processing steps performed after the cylindrical forming, a flange crack tends to occur at a HAZ (Heat Affected Zone) portion in the weld zone.
Nevertheless, the welding current needs to be relatively high to provide sufficient welding strength, thereby causing HAZ cracking.
Further, in recent steel can manufacturing processes, a coil coating process is carried out on steel sheets. It is desired to apply this coil coating method to steel for high-speed welding, but for this purpose it is necessary to form a non-varnished portion (not a coated portion) in parallel to the rolling direction and to arrange the winding direction of the can body in parallel to the rolling direction.
However, if the can body is wound in this direction and a flange forming process is performed thereafter, HAZ cracking is encountered. Accordingly, in the conventional art, the non-varnished portion (not the coated portion) has been arranged perpendicular to the rolling direction. As a result, high-speed welding could not be applied to the coil coated steel strip.
(4) Steel Sheet for Two-Piece Cans, and Its Problems
Conventional steel sheets for two-piece cans have been made from soft tempered tin mill black plate having excellent deep drawabilities. Further, since such a steel sheet was generally tin-plated, tin played a role as a lubricant during the process and the r-value was not required to be particularly large.
But in the case of using ultra-thin gauge and high strength steel sheet, since the r-value of the steel sheet is generally small, the drawability of the sheet was not desirable since portions around the bottom of the cup-shaped can cracked during the process.
In addition, the larger .DELTA.r-value (planer anisotropy of r-value) increases the earing phenomenon during cup processing, requiring the blank diameter to be uneconomically large.
Moreover, due to the lack of rigidity of the very thin steel sheet, creases occur on the can body wall during pressing, and cracking on the shoulder portion of the punch, respectively.
The same problems as in the hard raw sheet aforementioned took place in the DR raw sheet.
(5) Problem of Coating Weight
The steel sheet is generally subjected to tin-plating. Recently the coating weight of tin has been reduced to reduce cost. For example, while the conventional tin coating weight has been 2.8 g/m.sup.2, in the recent sheet that has sometimes been reduced to less than 1 g/m.sup.2. In such a case, the corrosion resistance of the steel sheet itself must be improved.
A great deal of effort has been made to cope with the foregoing problems, without success.
For example, Japanese Patent Publication No. Hei 1-52450 discloses a method for manufacturing steel sheets for T1-T3 cans by applying continuous annealing and thereafter temper rolling ultra low carbon steel. However, this method does not overcome all the aforementioned problems.